As an existing associate, do I have to sign-up again to take part in the Influencer Program?

No, you can use your existing Associates account and store ID to register for the additional benefits of the Influencer Program. Go to the Influencer Program landing page from Associates Central or any Influencer Page, click on "Join Now" and log into your Amazon account. If there is an Associates account linked to that Amazon account, you will be presented with the option to use your existing store ID (or one of several store IDs if you have more than one) to register for the program.